Laboratory Analysis Blood samples (approximately 3 ml from the cubital fossa of either arm) would be collected at birth, at 6 weeks and at 18 weeks of age of the recruited infant, by phlebotomists with relevant experience of drawing blood among infants. The blood samples would be analyzed for polio seroconversion rates. Neutralizing antibodies would be determined using the methods recommended by WHO [9]. The investigators will evaluate antibody responses to OPV by serotype in close collaboration with CDC (Atlanta). A subset of the samples will also be locally tested by ELISA to determine the sensitivity and specificity of ELISA as compared to the gold standard neutralization assays in detecting polio antibodies.